1. Field of the Invention
The present invention relates to a spark plug for igniting fuel in an internal combustion engine, particularly to a spark plug which is preferably used in a direct injection engine.
2. Description of Related Art
Conventionally, according to a spark plug, a center electrode and a ground electrode are arranged to interpose a spark gap therebetween and discharge is carried out at the spark gap by applying high voltage (for example, 30 kV) between the center electrode and the ground electrode.
Further, according to Japanese Patent Application laid-open No. 57-40886, an intermediate electrode comprising a semiconductor is installed in a midway of a spark gap mentioned above and discharge is carried out via the intermediate electrode. According to the conventional technology, when high voltage is applied between the intermediate electrode and the ground electrode, firstly, discharge is carried out at a first spark gap between the center electrode and the intermediate electrode by which the potential of the intermediate electrode is increased and accordingly, discharge is carried out successively at a second spark gap between the intermediate electrode and the ground electrode. Therefore, breakdown voltage (required voltage) at the spark gap can be lowered.
Meanwhile, according to the above-described discharge, inductive discharge is carried out after capacitive discharge (breakdown), and air is subjected to insulation breakdown by the capacitive discharge, and heat is provided to surrounding fuel by the inductive discharge by which growth of flame kernel is expedited. Further, breakdown voltage in the capacitive discharge is much higher than breakdown voltage in the inductive discharge and the breakdown voltage in the capacitive discharge is referred to as breakdown voltage at the spark gap.
According to the above-described conventional spark plug, the capacitive discharge and the inductive discharge are carried out via the above-described intermediate electrode and accordingly, flame kernel formed in the inductive discharge is liable to be brought into contact with the intermediate electrode and energy of the flame kernel is liable to be absorbed by the intermediate electrode. Accordingly, the growth of the formed flame kernel is hindered and ignition performance is lessened.
According to another conventional spark plug, it is normally fastened to an engine block of an engine by screws installed in the lower portion of a main body metal piece. However, the male screw of the main body metal piece of the spark plug and the female screw of the engine block cannot specify as far as the position of the screw thread with regard to the respective base materials. Accordingly, in mounting the spark plug to an engine, the direction of the ground electrode cannot be specified in the cylinder of the engine.
In the case of an engine directly injecting fuel into a cylinder, or the like which has been rapidly developed in recent years, a spray of gasoline moves in the cylinder. When a projected object such as the ground electrode of the spark plug or the like is disposed upstream from the spark gap in respect of the spray flow, the spray flow which is supposed to reach the spark gap is hampered by the projected object.
Particularly, the breakdown voltage and the ignition performance are significantly influenced by a relationship between a plane where the spark is flown and a direction of the spray flow.
In order to overcome the foregoing problem, setting the direction of the ground electrode in a cylinder may be possible by installing a nut to determine a positional relationship between the female screw of the engine and the male screw of the plug. However, a deviation is caused in an amount of projecting the plug into the cylinder by an amount of a plate thickness of the plate member or the pitch (for example, 1.25 mm) of the fastening screw.
In the case of a direct injection engine, the concentration of spray passing through the spark gap portion differs by the projection amount of the plug, and accordingly, the ignition performance is significantly influenced thereby.